This invention relates to blank gauges and more particularly to a unique thickness responsive blank gauge for indicating if a generally flat blank will properly wrap to a cylindrical shape of a predetermined inner diameter.
Heretofore, various problems have been experienced in the manufacture of electric motor shells. General industry practice has been to roll precut lengths of sheet metal into cylindrical form with the ends of the rolled sheet metal being welded. The internal diameter of the resulting cylindrical motor shell must be maintained within exact tolerances in order to permit the field laminations with electric motors to be press fit within the shell and to prevent distortion of the field laminations which result in defective motor operation.
In commonly owned U.S. Pat. No. 3,732,614 to Boutell, entitled METHOD FOR MAKING MOTOR SHELLS AND THE LIKE and issued May 15, 1973, an apparatus and method are disclosed for precisely forming cylindrical motor shells from flat blanks of sheet metal. A precisely dimensioned and precision-ground forming arbor or mandrel is used to produce the shells by wrapping individual precut lengths of sheet metal of an appropriate width around the forming arbor. Since the forming arbor has a diameter equal to the predetermined diameter of the field laminations for the electric motor, the cylindrically shaped motor shells will possess the required internal diameter and have accurate circularity.
The sheet metal stock from which the blanks are precut is typically supplied by manufacturers with a specified nominal thickness. The actual thickness of the individual cut blanks often varies within the tolerances allowed by the sheet metal manufacturers. This variance in thickness results in significant problems in the manufacture of motor shells. Variances in the thickness of the sheet metal blanks when wrapped around a precisely dimensioned arbor result in variance of the weld gap between the ends of the rolled or wrapped blank. If the thickness decreases from a nominal, stock thickness, the gap will decrease and the ends of the wrap blank may in fact overlap. If the thickness of the blank increases from the standard nominal thickness, the gap between the ends of the blank will increase thereby preventing an effective weld.
In U.S. Pat. No. 4,046,039 to the herein named inventor, entitled THICKNESS RESPONSIVE VARIABLE POSITION DIE SET and issued on Sept. 6, 1977, there is disclosed an apparatus for cutting metal or other sheet stock to a particular length which is determined by the stock thickness. The apparatus disclosed therein includes a stationary cutting block, a movable cutting block, stock thickness sensing apparatus and a bellcrank multiplier device for positioning the movable cutting block as a function of the sensed stock thickness. The variable position die set disclosed therein compensates for the variance in the thickness from a nominal or standard thickness and cuts the stock sheet to a cut length equal to the predetermined internal circumference plus pi times the thickness of the sheet.
Since the internal circumference of the cylinder must be capable of receiving a particular size of electric motor stator, having a given external diameter D, the cut length may be defined by the following equation: EQU L=pi (D+T)
since the initial thickness or nominal thickness T of any sheet metal stock will vary and the internal circumference of the resultant shell must be maintained constant, the relationship between the initial cut length L and subsequent cut lengths may be defined as follows: EQU L.sub.2 =L+pi.DELTA.T
where .DELTA. T equals the change in thickness between the different blanks. After initial calibration, the apparatus disclosed in the aforementioned patent will shift the movable cutting die set a distance proportional to pi times the change in thickness of subsequent sheets.
A need exists, however, for a relatively inexpensive and simple gauge which is capable of comparing a generally, flat blank from which the electric motor shells will be formed to a predetermined standard blank in order to determine if the cut length has been accurately determined. Such a gauge would have utility in checking the accuracy of the variable position die set disclosed in applicant's aforementioned patent and also in checking sheet metal blanks cut by more conventional techniques such as are disclosed in the aforementioned patent to Boutell.